The present invention relates to a fuel assembly for a boiling water reactor which is adapted, during operation of the reactor, to allow cooling water to flow upwards through the fuel assembly while absorbing heat from a plurality of fuel rods which are surrounded by a fuel channel, whereby part of the cooling water is transformed into steam, and wherein the fuel assembly comprises a steam channel through which the steam is allowed to flow through the fuel assembly towards the outlet end thereof.
In a boiling water reactor, moderated by means of light water, the fuel exists in the form of fuel rods, each of which contains a stack of pellets of a nuclear fuel arranged in a cladding tube. A fuel bundle comprises a plurality of fuel rods arranged in parallel with each other in a certain definite, normally symmetrical pattern, a so-called lattice, and is retained at the top by a top tie plate and at the bottom by a bottom tie plate. A fuel assembly comprises one or more fuel bundles, each one extending along the main part of the length of the fuel assembly and being surrounded by a substantially square fuel channel.
The core is immersed into water which serves both as coolant and as neutron moderator. The fuel assemblies are arranged vertically in the core and spaced from each other. During operation, the water is admitted through the bottom of the fuel assembly and then flows upwards through the fuel assembly past the fuel rods. The heat given off by the fuel rods is taken up by the water which starts boiling, whereby part of the water is transformed into steam. The water and the steam are passed out through the upper end of the fuel assembly. The produced steam is delivered to turbines which drive generators where electrical energy is generated.
A disadvantage with a boiling water reactor is the high percentage of steam in the upper part of the fuel assembly. When the percentage of steam rises in the coolant, its capacity to conduct heat away from the fuel rods is reduced, whereby the risk of dryout increases, which in turn leads to an increased risk of fuel damage. An additional problem with a high steam content in the fuel is that steam is much inferior to water as moderator, which has the consequence that the moderation becomes insufficient whereby the fuel is utilized inefficiently. In the lower part of the fuel assembly, the moderator consists of water whereas the moderator at the upper part of the fuel assembly consists of both steam and water. This means that the fuel in the upper part of the fuel assembly is not utilized efficiently. For this reason, it is desirable to reduce the steam content in the coolant while at the same time maintaining the steam generation at a high level.
The percentage by volume of steam a is a very important parameter for the nuclear properties of the fuel and the core. If the water phase and the steam phase both move at the same velocity, a may be directly determined from the mixing equation:                     α        =                  1                      1            +                                                            1                  -                  x                                x                            ·                                                ρ                  g                                                  ρ                  l                                                                                        (        1        )            
x=the percentage by weight of steam which is given by the supplied thermal energy
xcfx81g=the density of the steam phase
xcfx81l=the density of the liquid phase the quotient       ρ    g        ρ    l  
is about 0.05 for normal operating conditions in a boiling reactor at a pressure of about 7 MPa. The relationship between xcex1 and x is illustrated in FIG. 1.
If the steam phase and the liquid phase move at different velocities, this can be described with the following modification of equation 1:                     α        =                  1                      1            +                          S              ·                                                1                  -                  x                                x                            ·                                                ρ                  g                                                  ρ                  l                                                                                        (        2        )            
where S (the grinding factor i.e. slip factor) describes the velocity ratio between the phases, such as S greater than 1 means that the steam flows more rapidly than the liquid. S is actually a complicated function of the percentage by weight of steam, pressure, geometry, etc. It is well-known, however, that in a boiling reactor, a higher steam velocity is naturally obtained towards the outlet, which is due to the considerably lower density of the steam. If the steam flow has a higher velocity than the water flow, the percentage by volume of steam in the fuel assembly decreases. The real percentage by volume of steam is therefore 5-10% below that which could be expected without any velocity difference between the phases, at least in the upper part of the fuel assembly where the percentage by weight of steam is above 40%.
The conclusion of the above reasoning is that the faster the steam disappears from the fuel assembly, the lower will be the percentage by volume of steam. A separation of the steam flow and the water flow in the upper part of the fuel assembly thus gives the steam flow a higher velocity than the water flow, whereby the percentage of steam by volume in the fuel assembly decreases. In this way, the margin with respect to dryout is improved and the fuel in the upper part of the fuel assembly is utilized in a better way.
The patent specification of U.S. Pat. No. 5,091,146 shows a fuel assembly which attempts to achieve a separation of the steam flow and the water flow in the upper part of the fuel assembly by arranging a steam vent tube above one or more part-length fuel rods, that is, fuel rods extending from the bottom tie plate but terminating below and spaced from the top tie plate. In this way, the steam which is generated in the coolant is to be discharged. The tube has openings both in its upper and lower ends. The disadvantages of such a tube are several. For one thing, it is expensive to manufacture and, for another, it provides increased pressure drop in the fuel assembly. Another disadvantage is that it may be difficult to cause the continuously produced steam to enter the tube. Admittedly, the tube is provided with openings and other devices to encourage steam to flow into the tube and prevent water from entering the tube, but it is still doubtful whether this is an efficient way of causing the steam to enter the tube.
The invention aims to provide a fuel assembly which in a simple and efficient way separates the steam flow and the water flow at least partially, thus obtaining a lower percentage by volume of steam in the fuel assembly.
What characterizes a fuel assembly according to the invention will become clear from the discussion below.
A fuel assembly according to the invention comprises a vertical channel which conducts steam upwards through the fuel assembly during operation of the reactor. This channel has no walls but consists of only an empty volume between the fuel rods and will hereinafter be referred to as a steam channel.
The fuel assembly is designed such that the coolant, that is, water and steam, is brought to rotate around the steam channel so as to form an upwardly extending eddy. The eddy rotates so quickly that the steam is separated from the water with the aid of the centrifugal force. The water, which is heavier than the steam, is thrown outwards and away from the steam channel, whereas the lighter steam is pressed against the center of the eddy and hence against the steam channel. This gives the steam a considerably higher velocity than the natural one and allows the steam to leave the fuel assembly via the steam channel at a high velocity. In this way, the percentage by volume of steam in the fuel assembly is reduced.
The transport of steam to the center of the eddy intensifies the rotation since the moment of inertia decreases. Since the steam disappears out from the fuel assembly at a higher velocity than the water, the percentage by volume of steam in the coolant is reduced, which improves the cooling of the fuel. Also, a partial separation of water and steam is valuable. In practice, the first few percentage of reduction of the steam volume provides the greatest yield.
The invention affords a plurality of advantages. Most of them originate from the reduced percentage by volume of steam. The cooling is improved and hence the margin with respect to dry-out increases. A smaller amount of steam increases the reactivity whereby the need of enriching the fuel is reduced. The shutdown margin is improved since less reactivity is bound in the steam. A reduced variation in the axial percentage by volume of steam and the reduced number of fuel rods upwards in the bundle make it possible to achieve an essentially optimum water/uranium ratio along the whole length of the assembly, which increases the reactivity further. The more open lattice and the smaller percentage by volume of steam provide a lower pressure drop. Less reactivity bound in the steam leads to less negative reactivity coefficients. The stability of the core and several different transients are improved.
Still another advantage is the improved shutdown margin, which is a consequence of the upper part of the fuel containing at least one coherent region without fuel rods. During shutdown, these are water-filled and contribute to considerably reduce the reactivity and hence improve the shutdown margin.